Rheostat or Potentiometer for Stirplate?

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rrittenhouse

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Hello EE's I have a question. I am planning on building a stir-plate and I am seeing designs with a rheostat and others with circuits that were built with a couple components.

Are there any differences/advantages/disadvantages in these designs?

Thanks
 

rekoob

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if your going with a DC fan to make your stir plate, you can just use a POT to very the 12 volts to the fan.

not much difference between the two, the POT has a wiper arm, the rheostat has two terminals. either will work for you case, not sure what the price difference is for them. what plans are you following for your build? I suppose that will decide.
 
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rrittenhouse

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The latest plans with the rheostat are located here: http://www.wortomatic.com/articles/Building-a-Basic-Stirplate-for-Yeast-Suspension

I guess I've read that you need the rest of the circuitry if you go with the pot instead of the rheo but I wanted to confirm this. If its that easy i'm just going to use the rheo. I was told the pots can only handle about 0.25W or so. I am just trying to clear this all up so I can build one.

Thanks again
 

a10t2

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If you're going to use just a pot you need to get one that's rated for at least 2W (assuming you'll be using a 12 VDC power supply). You can also use an LM317 to dissipate the current, in which case the power rating for the pot doesn't really matter. The only additional components needed to put together a basic LM317 circuit would be a resistor and two capacitors. http://seanterrill.com/2010/04/26/build-a-better-stirplate/
 
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rrittenhouse

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Ok. So unless theres a huge price difference I can just get the rheostat or a 2W pot. As much as I like to tinker sometimes its just nice to simplify things.....at first lol. I'm sure i'll mod it later and add some un-necessary features.

Thanks for the response.
 

Sawdustguy

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If you get a 2 watt Pot you can simply wire as a rheostat. You can use a simple 12 VDC 1 Amp wallwart as your voltage source. See below.....

 
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rrittenhouse

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Does it really need to be a 2W potentiometer? Not that I cant find one but when I do the power calculation is around 50mW. Where did I go wrong?
 

a10t2

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If your fan motor is pulling ~150 mA (roughly average from the ones I've seen), using a 12 V supply, and you want to run the motor at 4 V, the power dissipation in the resistor is:

(12 - 4)(0.15) = 1.2 W

The slower the fan is turning, the more power dissipated in the pot as heat.
 

rekoob

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actually, Power=current times voltage. P=IE. so running at 4 volts would be .6 given your current draw. you would have to factor in the resistance to know what the current draw is overall. go with what ever you want as in a pot or a rheostat, if your using a CPU fan for your build its not going to draw enough to make any difference. the higher you get in resistance the less of the "throw" you will use, meaning half the "turns" might not change the speed of the fan.
 
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